Mercury has apparently shrunk more than previously thought

Mercury, the planet closest to the sun, has apparently shrunk considerably more than astronomers previously expected. This is shown by a new analysis of images of the planet's surface by a research team from Japan and Germany.

The planet's diameter may have decreased by more than 23 kilometers since its formation, the scientists report in the journal Geophysical Research Letters. Until now, the researchers had assumed a maximum of 14 kilometers.

How much Mercury has shrunk provides important insights into the internal structure and development of the planet. Greater shrinkage indicates a larger metal core, explains Gaku Nishiyama from the Institute for Space Research at the German Aerospace Center. “Other possibilities would be a lower proportion of light elements in the metal core or a higher temperature during its formation.”

Mercury, like the other planets in our solar system, was formed 4.5 billion years ago from condensations in a rotating disk of gas and dust around the young sun. First, smaller bodies formed, from which the large planets eventually formed through collisions and mergers. The collisions released a lot of energy. The resulting planets were initially hot and liquid.

On the surface, the inner planets formed a solid crust made of lighter rock that floated on the liquid magma. Over billions of years, the planets slowly cooled - and shrunk.

Although this shrinkage is less than one percent compared to the diameter of a planet, it causes enormous stress in the crust. The result: Mercury's surface is covered in folds and wrinkles. Or, to put it more technically, there are fractures and mountain ranges everywhere that were formed by the crust being pushed together.

From these traces of shrinkage, planetary scientists had deduced a decrease in Mercury's diameter by up to 14 kilometers. But there is a problem: The shrinkage should be uniform everywhere on the planet - but the distribution of folds and wrinkles is very uneven, as images from the American space probe “Messenger” show.

While some regions show a large number of such structures, others are almost free of them. Why is that? Nishiyama and his colleagues assume that meteorite and asteroid impacts have erased traces of shrinkage in many regions.

For their new calculation, the researchers only took into account those regions that were spared from this influence. This means the team can achieve a change in diameter of up to 23 kilometers. But even this value could underestimate Mercury's shrinkage, emphasizes Nishiyama.

The analyzes of the messenger data primarily record larger, connected faults with a length of around five kilometers. So there could be many smaller, previously overlooked signs of shrinkage. That could change soon. On November 21st of this year, the European probe BepiColombo will enter an orbit around Mercury.

BepiColombo is intended to map Mercury completely with a resolution of 50 meters. A special camera can also provide images of selected regions that show details with a size of five to six meters. This should give the researchers definitive information about the shrinking of Mercury - and thus also gain new insights into the interior of the planet.